1. Field of the Invention
This invention is related to electroless deposition processes and more specifically to methodologies for creating particle enhanced electric joining surfaces through modified electroless deposition processes.
2. Description of Related Art
Electroless nickel bumping (ENB) of contact surfaces is currently a primary technique for providing electrical, thermal, and mechanical connections for integrated circuit chips. The time required to bump a chip by electroless plating is governed by the rate of the metal deposition reaction and the desired height of the bump. Therefore, there is a practical limitation to the throughput of chips in the manufacturing process. Once a chip is bumped, several other steps are necessary to attach the chip to an opposing contact surface. First, oxidation on the opposing contact surface must be removed in order to provide a good electrical connection. Second, in order to attach a bumped chip to an opposing contact surface, the contact joint must be heated and the bumped chip either soldered to the opposing contact surface, or heated enough that the bump reflows to join the contact surface of the chip with the opposing contact surface. In the alternative, a conductive adhesive may be used to attach the bumped chip to the opposing contact surface. These additional steps needed to ensure a good electrical and mechanical connection inject added time and cost into the manufacturing process.
DiFrancesco in U.S. Pat. No. 5,083,697 originally disclosed particle enhancement of contact surfaces to provide improved electrical, thermal, and mechanical connections between contact surfaces. DiFrancesco suggested that the particle enhanced contact surfaces may be formed by employing a variety of techniques, such as electroplating, electroless plating, chemical vapor deposition (CVD), sputter deposition, and evaporation. Many of the methods suggested by DiFrancesco have practical disadvantages. For example, a major disadvantage of electroplating particles to enhance contact surfaces is that the method is not suitable if many electrically isolated contact surfaces are to be coated, for example, the contact surfaces of IC chips.
With particular references to electroless plating, DiFrancesco did not anticipate the following problems. First, metallized conductive particles are incompatible with an electroless plating solution, as metallized particles cause instant solution decomposition because of the large collective surface area of the metallized particles in plating solution. Second, direct deposition of nonconductive particles by conventional electroless plating methods is proven through experimentation to result in poor electrical conductivity because of poor metal coverage on the particles.
The plating of particulate matters onto metal surfaces and other substrates has been commonly practiced since 1960s. The aim of conventional composite electroless plating is to successfully co-deposit particulate matter in a metal film to create a new composite. These electroless composite processes are generally designed to provide a regenerated surface layer for the purpose of corrosion resistance or lubrication. In a conventional composite deposition, significant xe2x80x9cmechanicalxe2x80x9d effort is devoted to ensure either a successful xe2x80x9cinclusionxe2x80x9d type of co-deposition of lubricating particles, such as PTFE and graphite fluoride, or a xe2x80x9cstructuralxe2x80x9d composite for the purpose of improving corrosion resistance of an electrodeposited structure. Therefore, most particles (95% or more of the deposited particles) in such composite depositions must be densely structurally implanted (without large xe2x80x9cblack-holexe2x80x9d type defects) in a metallic matrix film. These metal films must be of a certain thickness. Typically the thickness is of at least one order of magnitude greater than the average particle size, in order to achieve the anti-corrosion or lubricating benefits. The conventional electroless composite deposition process is not designed to provide an electrical or mechanical connection between two opposing surfaces, nor does it result in such benefits in actuality.
The present invention provides a unique method for co-depositing hard particles and metal on electrical contact surfaces to provide mechanical, thermal, and electrical connections between the contact surfaces, and to enhance the thermal and electrical conductivity between the contact surfaces and their corresponding substrates. The innovative method is able to uniformly deposit metal and particles of any shape, and with a wide range of density and sizes, on contact surfaces, and can be adjusted to provide any desired surface area coverage in desirable deposition patterns. The co-deposited contact surface can, for example, be easily joined to another surface of any type by nonconductive adhesive, resulting in a connection that is mechanically robust, chemically inert, and inherently electrically conductive. This eliminates the necessity of using specialized conductive adhesive or extreme heat for soldering or bump reflow for creating electrical surface joints.
In the relevant art, the term xe2x80x9csubstratexe2x80x9d is used interchangeably to indicate a wafer, an integrated circuit chip, a contact pad on a chip, a circuit board, and various other dielectric and conductive materials and surfaces. In order to avoid potential confusion, in this specification the term xe2x80x9ccontact surfacexe2x80x9d is used to indicate that portion of a substrate through which external electrical connections are made. A contact surface would therefore include the contact pads or area array contacts on a wafer, chip, or other substrate, but would not refer to the chip or wafer itself, or other elements thereof. While the embodiments discussed herein focus on the use of the inventive process to co-deposit metal and particles on contact surfaces, this is not meant to indicate any limitation of the process to such surfaces, and indeed the co-deposition process can be applied to other surfaces capable of accepting electroless plating as well.
The present invention consists of a modified two-step electroless metal plating process. The first plating step utilizes a modified composite electroless metal plating method to co-deposit metal and particles on a contact surface, wherein the preferred metal is nickel. A particle surface activation step is performed after the co-deposition to ensure adhesion of the metal to exposed particle surfaces during the second metal plating. At this time the first layer of metal may also be activated to achieve a more efficient metal deposition in the second plating process. The second plating step is a conventional electroless metal plating process, again preferably using nickel. The unique process disclosed herein is designed to produce a consistent and uniform dispersion of hard particles on a contact surface through modified electroless plating techniques. The deposits typically contain 5% to 50% by weight of occlusal hard particles, which uniformly cover the contact surface. Higher and lower surface density implantations of particles can be obtained to match any specific applications.
The second electroless metal plating step places a layer of metal film on the hard particles that have already been deposited by the first electroless metal-particle co-deposition process. The second metal layer does more than ensure the bonding strength the implanted particles need. More importantly, it also provides a conductive overcoat on the particles, which allows originally nonconductive particles to function as electrically conductive media. Without the second electroless metal plating, the metal and hard particle deposit of the first deposition often results in inferior electrical conductivity, since the metal coverage on the hard particle outcrops is often poor. Also, directly using conductive particles in an electroless metal deposition process can cause instant decomposition of the plating solution and particle deposition fails.
The disclosed two-step electroless metal plating process using the preferred metal, nickel, provides a uniform and less porous metal layer to most configurations of substrate. This provides a stronger bond for the particles and thereby a superior force enhancement for electric contact, i.e., less force is needed to establish conductivity when mounting a particle enhanced component because of the high force per unit area transferred to the particles, which embed in the contact surface.
The two-step electroless metal process proposed in the present invention also offers a much more uniform and desirable particle deposition pattern on contact surfaces. Because the particle deposition results from an electroless plating process, the particles are not subject to the influence of any electric fields, which is inevitable in an electrolytic co-deposition process. No particle treeing and little clustering is observed. As a result, generally uniform, single-layer hard particle deposition is achievable at little extra expense over a normal electroless plating process.
In actual application with common electrical components, the contact surfaces prepared by the present electroless metal-particle co-deposition process offer the following benefits over traditional metal bumping techniques and other particle enhancement techniques. First, the present invention is ideal for preparing semiconductor wafers for flip chip attachment. All the contact surfaces on the wafer can be treated in a single step without need for electrical bridging.
Second, the methodology provides for the application of nonconductive particles to create a conductive media. The second nickel-plating step deposits an overcoat on the nonconductive particles, which provides an electrical pathway between the joined contact surfaces. This essentially enables any type of hard particles compatible with electroless plating solutions to be used as contact surface joining media, especially a large selection of nonconductive particles, such as most ceramics, glasses, oxides, silicates, nitrides, and diamonds. Many particles in these categories are manufactured in other industries and are commercially available in large quantities and consistent quality. For instance, diamond powders are widely used as abrasive materials.
Third the electroless process results in a desirable contact surface with a designable particle deposition pattern. By a designable particle deposition pattern, it is meant that through the choice of particle material and size, particle concentration in the plating solution, and duration of the plating treatments, the height of the particle bump on the contact surface, the thickness of the metal coating on the particles, the surface density of the particles on the contact surface, and the thermal conductivity of the bond can all be controlled and modified to meet particular specifications for particular applications. For example, the two-step electroless metal-particle plating method enables the production of flat contact surfaces with single layer, well aligned, non-clustering, non-treeing, uniformly dispersed, particle patterns on the contact surfaces, which is ideal for contact surface joining and is suitable for wide applications, such as smart cards and LED arrays.
Fourth, the particle enhanced contact pads create improved electrical, mechanical, and thermal performances for surface electric connections. Two-step electroless nickel-particle plating, by nature, provides a denser and more mechanically robust co-deposition structure than, for example, electrolytic plating. The second nickel plating and immersion gold steps offer electrical pathways with a very low resistance. The particle-enhanced surfaces also provide exceptionally short circuit pathways aiding the low resistance of the electrical connections. Further, particle enhanced contact surfaces require neither solders nor conductive adhesives, which are often required by chips bumped by electroless nickel. Mechanically, the hard particles have the ability to pierce any nonconductive surface barrier, for example an oxide layer on either or both the co-deposited substrate and the opposing contact substrate, providing good electrical connection without the need for removal of the oxidation or other barrier. In addition, hard particles, such as diamonds, provide excellent thermal conductivity between joined surfaces. The improved thermal conductivity results because of the greater surface area provided by a multiplicity of rugged particles per bond site (typically 5 micron industrial diamond has a surface area of about 1 m2/g) and higher coefficient of thermal conductivity for particles (typically around 10 W/cm/K for the industrial diamonds used) than typical bumped bonds. Altogether, the electroless co-deposition process offers perhaps the best combination of benefits available compared to current technology for joining electrical contact surfaces.